2,2′-Biphenol-Derived Phosphoric Acid Catalyst for the Dehydrative Esterification of Carboxylic Acids with Alcohols

Dedicated to Professor Hisashi Yamamoto on the occasion of his 80^th birthday

Abstract

A dehydrative esterification from an equimolar mixture of carboxylic acids and primary or secondary alcohols in toluene at 100 °C was promoted without the necessity to remove water by using a simple 2,2′-biphenol-derived phosphoric acid catalyst (2.5–10 mol%). This reaction was also successfully conducted at the gram scale. To demonstrate the synthetic utility of this catalytic system, pharmaceutically useful substrates and acid-sensitive substrates were examined using these acid–base cooperative phosphoric acid catalysts, which exhibit relatively weak Brønsted acidity.

Key words

alcohols - carboxylic acids - DFT calculations - esterification - green chemistry - organocatalysts - phosphoric acid - pK _a

Publication History

Received: 21 April 2023

Accepted after revision: 23 May 2023

Article published online:
10 July 2023

• References and Notes


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• 23 Since catalysts 1e and 1h were soluble under our reaction conditions at 80–100 °C, the yields of the products in Scheme 6 might not simply depend on the solubility of the catalysts. At the present preliminary stage, we cannot clearly explain the observed results in Scheme 6, and pK _a value, the basicity of the P=O moiety, homodimerization, and conformational flexibility of the catalysts and the instability of the starting materials and products might complicate the results.
• 24 General Procedure: A thoroughly flame-dried, two-necked flask with a condenser was charged with catalyst 1h (12.4 mg, 0.050 mmol, 5 mol%) under an argon atmosphere. Toluene (2 mL), carboxylic acid 2 (1.0 mmol), and alcohol 3 (1.0 mmol) were added at room temperature. The mixture was then stirred at 100 °C (bath temperature) for the indicated reaction time (6–72 h), and the reaction was monitored by TLC. Subsequently, the mixture was cooled to room temperature and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (n-hexane/EtOAc, 20:1 to 5:1) to give 4. For characterization data and copies of NMR spectra, see the Supporting Information.